1. Field of the Invention
The present invention relates generally to the field of electronics, and specifically to the aspects of battery charging, more particularly in relation to the use of the USB (Universal Serial Bus) interface for the charging of portable or mobile, battery-operated devices.
2. Discussion of the Related Art
In the last few years, portable and mobile, battery-operated digital electronic devices like mobile (cellular) phones, digital cameras, camcorders, PDAs (Personal Digital Assistants) have experienced a massive diffusion through users.
Mobile electronic devices use significant battery power; for example, the several different and more and more complex multimedia functions that are currently implemented by a mobile phone have a significant impact on the device's power consumption. Thus, the device battery should be re-charged as quickly and as often as possible.
Thanks to the success gained by the USB as a means for the connection of Personal Computers (PCs) to PC peripherals like printers, keyboards, pointing devices, memory card readers, pen drives, just to cite some, the producers of consumer electronics devices have been driven to equip also portable and mobile devices with USB interfaces. The presence of such USB interfaces may be advantageously exploited for charging the portable or mobile device battery in an alternative way with respect to the employment of AC/DC adapters.
More particularly, the battery of a portable or mobile device may be charged through the USB interface by exploiting the USB current and the USB voltage supply provided by whichever electronic device that is adapted to operate as a USB host (i.e., an electronic apparatus, like a PC, to which a USB device acting as a peripheral thereof can be connected), or by a USB wall adapter, for the connection to an AC-main socket.
Usually, USB hosts are capable of providing currents at most equal to 500 mA, while USB wall adapters may guarantee higher maximum currents, up to 1,5 A or more.
Since the USB voltage supply is a voltage in the range from approximately 4.85 V to approximately 5.25 V, and since battery-operated electronic devices are typically powered by a lithium (lithium ion or lithium polymer) battery, which provides a voltage in the range from approximately 3.0 V to approximately 4.2 V, a buck charger including a DC-DC voltage down-converter needs to be embedded in the battery-operated device.
DC-DC voltage down-converters are commonly of the switched-mode type. In this case, the down conversion of the USB voltage supply is carried out by applying a DC voltage across an inductor or transformer for a period of time which causes current to flow through it and store energy magnetically, then switching this voltage off and causing the stored energy to be transferred to the voltage output in a controlled manner.
In order to regulate the charge operations, it is possible to use one among the methods already known in the art. The known methods are all based on the monitoring of the current that is injected into the battery to be charged, i.e., the current that is provided by the DC-DC voltage down-converter to the battery. In particular, the known methods provide for monitoring and regulating the current comparing it with the maximum current that the USB host is able to deliver to the battery through the DC-DC voltage down-converter.
However, these methods present several drawbacks, that cause the charge of the battery to be slow and scarcely efficient.
Indeed, the current that is injected into the battery to be charged by the DC-DC voltage down-converter depends on the current provided by the USB host through a relationship that depends on the efficiency of the DC-DC voltage down-converter and the voltages of the input and output thereof; since the values of the quantities are not know a priori, by merely sensing the current that is injected into the battery it is not possible to estimate the value of the USB current in a precise way. The impossibility of estimating the USB current value may have detrimental effects on the performance of the charge operations.
In particular, since this solution does not allow regulating the USB current—whose value remains unknown—, it may happen that the battery is charged using a charging current that is significantly lower than the maximum USB current deliverable by the USB host.
A further drawback caused by the adoption of this solution regards the possibility of connecting more than one battery-operated device to be charged in parallel to the same USB host. Indeed, in this case, the sum of all the currents drained by the devices may exceed the maximum USB current that the USB host is capable of delivering, since the actual values of the currents drained by each device are not known.
Thus, the Applicant has found that by directly monitoring the current (generated by the USB host) that is injected into the DC-DC voltage down-converter it is possible to noticeably increase the performance of the charge operations.